Nuclear reactor coolant is coolant in a nuclear reactor that is used to remove heat from the nuclear reactor core and transfer it to electrical generators and the environment. Frequently a chain of two coolant loops is used because the primary coolant loop takes on short-term radioactivity from the reactor.

Almost all currently operating nuclear power plants are light water reactors using ordinary water under high pressure as coolant and neutron moderator. About 1/3 are boiling water reactors where the primary coolant undergoes phase change to steam inside the reactor. About 2/3 are pressurized water reactors at even higher pressure. Current reactors stay under the critical point at around 374 °C and 218 bar where the distinction between liquid and gas disappears, which limits thermal efficiency, but the proposed supercritical water reactor would operate above this point.

Fast reactors have a high power density and do not need neutron moderation. Most have been liquid metal cooled reactors using molten sodium. Lead and other metals have also been proposed and occasionally used.

Molten salts share with metals the advantage of low vapor pressure even at high temperatures, and are less chemically reactive than sodium. Salts containing light elements like FLiBe can also provide moderation. In the Molten-Salt Reactor Experiment it even served as a solvent carrying the nuclear fuel.

Gases have also been used as coolant. Helium is extremely inert both chemically and with respect to nuclear reactions but has a low heat capacity, necessitating rapid circulation. Carbon dioxide has also been used. Gases of course need to be under pressure for sufficient density at high temperature.

Nuclear reactor coolants

Coolant	Melting point	Boiling point
Light water at 155 bar		345 °C
Mercury	-38.83 °C	356.73 °C
NaK eutectic	-11 °C	785 °C
Sodium	97.72 °C	883 °C
FLiBe	459 °C	1430 °C
Lead	327.46 °C	1749 °C
Lead-bismuth eutectic	123.5 °C	1670 °C

The Tōkai Nuclear Power Plant was Japan's first nuclear power plant. It was built in the early 1960s to the British Magnox design, and generated power from 1966 until it was decommissioned in 1998. A second nuclear plant, built at the site in the 1970s, was the first in Japan to produce over 1000 MW of electricity, and still produces power as of 2009. The site is located in Tokai in the Naka District in Ibaraki Prefecture, Japan and is operated by the Japan Atomic Power Company.

Tokai Nuclear Power Plant Reactors

Unit	Type	Average electric power	Capacity	Construction started	Construction completed	First criticality	Closure
Tōkai I	Magnox (GCR)	159 MW	166 MW	March 1, 1961	November 10, 1965	July 25, 1966	March 31, 1988
Tōkai II	BWR	1060 MW	1100 MW	October 3, 1973	March 13, 1978	November 28, 1978

Unit 1

This reactor was built based on British developed Magnox technology. Unit 1 will be the first nuclear reactor to be decommissioned in Japan. The experience in decommissioning this plant is expected to be of use in the future when more Japanese plants are decommissioned. Below is a brief time-line of the process.

• March 31, 1998: operations cease
• March 2001: last of the nuclear fuel moved off-site
• October 4, 2001: decommissioning plan announced
• December 2001: decommissioning begins, spent fuel pool is cleaned
• 2003: turbine room and electric generator taken down
• Late 2004: fuel moving crane dismantled
• 2011: the reactor itself is dismantled

Tokai Nuclear Power Plant Unit 2

This Boiling Water Reactor was the first nuclear reactor built in Japan to produce over 1000 MW of electricity. By some formalities in the paperwork, the unit is technically separate from the rest of the nuclear facilities at Tokai-mura, but it is managed with the rest of them and even shares the same front gate. The power produced at the unit is sold by both the Tokyo Electric Company and the Tohoku Electric Company.

Tokai Nuclear Power Plant After 2011 Tohoku Earthquake and Tsunami

The number 2 reactor at Tōkai Nuclear Power Plant was shut down automatically. On 14 March it was reported that a cooling system pump for this reactor had stopped working; however, the Japan Atomic Power Company stated that there was a second operational pump sustaining the cooling system, but that two of three diesel generators used to power the cooling system were out of order.

According to the ministry, a total of 11 nuclear reactors were automatically shut down at the Onagawa plant, Fukushima No. 1 and No. 2 plants and Tokai No. 2 plant after the biggest-magnitude quake in the country's modern history.

The Fukushima prefectural government advised some 2,000 residents in a radius of 2 kilometers of the No. 2 reactor of the Fukushima No. 1 plant to evacuate, government officials said. The officials said the operator, Tokyo Electric Power Co., reported the level of water going down in the reactor.

Tokyo Electric also said the system to cool reactor cores in case of emergency stopped at the No. 1 and No. 2 reactors of its Fukushima No. 1 nuclear plant.

At the Onagawa plant in Miyagi Prefecture, a fire started at a building housing the turbine, the operator, Tohoku Electric Power Co., said, denying it detected any signs of radiation leaks. Water spilled from pools containing fuel rods at the Kashiwazaki-Kariwa plant on the Sea of Japan coast in Niigata Prefecture and the Onagawa plant.

Hokkaido Electric Power Co. reported no problems at its Tomari No. 1, No. 2 and No. 3 plants on the northernmost main island.

There were no immediate signs of any problems at the Hamaoka nuclear plant on the Pacific coast in Shizuoka Prefecture, southwest of Tokyo, the prefectural government said.

The Higashidori Nuclear Power Plant is a nuclear power plant located in the town of Higashidōri in eastern Aomori Prefecture, on the Shimokita Peninsula, facing the Pacific Ocean. It is unique in Japan in that at this four-unit site, two units are run by one company, the Tōhoku Electric Power Company and two units are run by the Tokyo Electric Power Company. The reactors are all of Toshiba design.

Construction of Tohoku Electric's Higashidori Unit-1 began in December 1998 and was completed in December 2005. The design was based on Tohoku Electric's Onagawa Nuclear Power Plant Unit-3, with improvements to the reactor vessel to allow for greater ease in inspection and maintenance. A separate building, apart from the containment structure, is dedicated specifically for the heat exchanger system based on seawater to provide primary coolant for the reactor.

Reactors Run by Tohoku Electric Company

Unit	Type	First Criticality	Electric Power
Higashidori - 1	BWR	December 8, 2005	1100 MW
Higashidori - 2 (planned)	ABWR	Construction will start 2010	1385 MW

Reactors run by Tokyo Electric Company

Tepco began construction of its Higashidori-1 unit on January 25, 2011, after approval by METI.

Unit	Type	First Criticality	Electric Power
Higashidori - 1 (planned)	ABWR	scheduled for completion in March 2017	1385 MW
Higashidori - 2 (planned)	ABWR	scheduled for completion in 2019	1385 MW

Higashidōri Nuclear Power Plant
Country	Japan
Construction began	2000
Commission date	December 8, 2005
Operator(s)	Tōhoku Electric Power Company Tokyo Electric Power Company
Reactor information
Reactors operational	1 x 1100 MW
Reactors planned	3 x 1385 MW
Power generation information
Annual generation	9,269 GW·h
Net generation	12,197 GW·h

The Onagawa Nuclear Power Plant is a nuclear power plant in Onagawa in the Oshika District and Ishinomaki city, Miyagi Prefecture, Japan. It is managed by the Tohoku Electric Power Company. It was the most quickly constructed nuclear power plant in the world.

The Onagawa-3 unit is the most modern reactor in all of Japan. It was used as a prototype for the Higashidori Nuclear Power Plant. The plant conforms fully to ISO 14001, a set of international environmental management standards. The plant's waste heat water leaves 7 degrees Celsius higher than it came in and is released 10 meters under the surface of the water, in order to reduce adverse effects on the environment.

All the reactors were constructed by Toshiba.

Onagawa Nuclear Power Plant Reactors

Unit	Type	Start of Operation	Electric Power
Onagawa - 1	BWR	June 1, 1984	524 MW
Onagawa - 2	BWR	July 28, 1995	825 MW
Onagawa - 3	BWR	January 30, 2002	825 MW

Unit 1

Since November 11, 2006 this unit has been shut down due to the result of a test.

Unit 2

• May 2006 it was confirmed that a pipe was leaking due to debris damage.
• June 7, 2006 Difficulties with pressure control prompted further inspections.
• July 7, 2006 METI and the Nuclear and Industrial Safety Agency determined that the plant's performance was not satisfactory.

Unit 3

• July 7, 2006 Due to pipe integrity concerns the reactor was shut down.
• November 25, 2006 Following repairs the reactor was restarted.

Earthquake Events

• The Onagawa Nuclear Power Plant was affected by the 2005 Miyagi earthquake and recorded vibrations above what the plant was designed for. Analysis after the event, however, found no damage to the reactor systems. Some people reported seeing smoke come from the plant after the earthquake and reported it, thinking that it indicated an accident, but the smoke was actually produced by the backup diesel generators. These generators were designed to start up in such an event so that off-site power can still be provided to certain reactor systems. A tank of sulfuric acid on the roof did burst releasing some sulfuric acid into the air, but it was retrieved and processed later.
• A fire at the plant from the turbine section of the plant following the 2011 Sendai earthquake was reported by Kyodo News.
• On 13 March the lowest-level state of emergency was declared regarding the Onagawa plant by TEPCO, as radioactivity readings temporarily exceeded allowed levels in the area of the plant. TEPCO stated this was due to radiation from the Fukushima I nuclear accidents and not from the Onagawa plant itself.

Onagawa Nuclear Power Plant
Country	Japan
Construction began	1970
Commission date	June 1, 1984
Operator(s)	Tohoku Electric Power Company
Reactor information
Reactors operational	1 x 524 MW 2 x 825 MW
Power generation information
Annual generation	5,283 GW·h
Net generation	157,545 GW·h

A fire broke out in the turbine building of Onagawa Nuclear Power Plant after the 8.9 magnitude earthquake that struck Japan today and triggered a huge tsunami. There is also a fire at a nuclear reactor which is burning out of control.

The Japanese government has declared a nuclear power emergency situation following Friday's devastating earthquake, although the prime minister, Naoto Kan, said that there had been no reports of radiation leaks at any of the country's nuclear facilities.

The chief cabinet secretary, Yukio Edano, said the nuclear power plant in Fukushima developed a mechanical failure in the reactor cooling system after it was shut down during the earthquake.

He said the measure was a precaution and there was no radiation leak at the Fukushima No 1 power plant. He said the facility was not in immediate danger.

"Parts of nuclear plants were automatically shut down but we haven't confirmed any effects induced by radioactive materials outside the facilities," Kan said.

The UN nuclear watchdog in Vienna saidthe four Japanese nuclear power plants closest to the earthquake have been safely shut down.

Tokyo Electric Power said the power had been cut to 4 million homes. Among other reports of damage, Japanese media reported a fire at JFE Holdings' steel plant in Chiba. JFE, the world's fifth-largest steelmaker, said there was no major impact. Cosmo Oil said its Chiba refinery had been hit by a fire in storage tank, and it was still working to extinguish it. JX Nippon Oil, Japan's top refiner, halted operations at three refineries in Sendai, Kashima and Negishi, Jiji News said. Television showed black smoke pouring out of an industrial area in Yokohama's Isogo area.